Memory System

Thursday, June 18, 2009

The main characteristic of memory is its capacity. Internal memory capacity is typically expressed in terms of bytes or words. Common word lengths are 8-bit, 16-bit and 32-bits. External memory capacity is typically expressed in terms of bytes.

• Word – the natural unit of organization of memory. The size of the word is typically equal to the number of bits used to represent a number. • Addressable units – word or byte.• Unit of transfer – number of bits read out of or written in to memory at a time.

We can divide memory systems based on method of accessing. There are

• Sequential Access – access made in a specific linear sequence. A shared read/write mechanism is used.• Direct Access – individual blocks or records have a unique address based on physical location. Access is accomplished by direct access followed by sequential access. Shared read/write mechanism is used.• Random Access – each addressable location in memory has a unique. Physically wired in addressing mechanism.• Associative Access – a random access type memory that enables one to make a comparison of desired bit locations within a word for specified match, and to do this for all words simultaneously. Thus a word is retrieved based on a portion of contents rather than its address.

• Access time (Latency) – time taken to perform a read/write operation.• Memory cycle time – access time plus any additional time required before a second access can commence (applicable to RAM)• Transfer time – rate at which data can be transferred into or out of memory unit.

For RAM = 1/ (cycle time)

Non RAM TN = TA + N/R

TN = average time to read or write N bitsTA = average access timeN = number of bitsR = transfer rate in bits per second

• Volatile memory Information decays naturally or is lost when electrical power is switched off• Non volatile memoryInformation one recorded remains without deterioration until deliberately change. No electrical power is needed to retain information.